There is often a need in integrated circuits to have a reliable source for a reference voltage. One widely used voltage reference circuit is the band-gap voltage reference. The band-gap voltage reference is generated by the combination of a Proportional to Absolute Temperature (PTAT) element and a Complementary to Absolute Temperature (CTAT) element. The voltage difference between two diodes is used to generate a PTAT current in a first resistor. The PTAT current typically is used to generate a voltage in a second resistor, which is then added to the voltage of one of the diodes. The voltage across a diode operated with the PTAT current is the CTAT element that decreases with increasing temperature. If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature can be largely cancelled out, providing a more or less constant voltage of about 1.2-1.3 V, depending on the particular technology.
Since band-gap circuits are often used to provide an accurate, temperature independent reference voltage, it is important to minimize the voltage and temperature related variations over the likely temperature range over which the band-gap circuit will be operated. One usage of band-gap circuits is as a peripheral element on non-volatile memory circuits, such as flash memories, to provide the base value from which the various operating voltages used on the circuit are derived. There are various ways to make band-gap circuits less prone to temperature dependent variations; however, this is typically made more process limited, and is difficult in applications where the band-gap circuit is a peripheral element, since it will share the same substrate and power supply with the rest of the circuit and will often be allowed only a relatively small amount of the total device's area.